Tiny membrane converts radio waves to light

A device that detects ultra-weak radio waves by converting them into light signals has been created by physicists in Denmark and the US. The device does not require costly cryogenic cooling and could be put to practical use in a range of applications, from radio astronomy to magnetic resonance imaging. The researchers also believe that the technology could provide an essential building block of a "quantum internet" of the future.

Detecting extremely weak radio waves is at the heart of many modern technologies, including satellite navigation, long-distance communications, radio telescopes and magnetic resonance imaging (MRI) systems. In some detectors, weak radio signals are converted into optical signals that can then be transported long distances via optical fibres. In addition to requiring expensive modulators to convert the electronic signals into optical signals, these converters must be cooled to cryogenic temperatures, making them expensive and inconvenient to operate.

The new device was created by Eugene Polzik and colleagues at the University of Copenhagen, along with researchers at the Technical University of Denmark and the Joint Quantum Institute at the University of Maryland. The team says that its device can detect extremely weak radio waves by converting them directly into light signals. These signals can then be transmitted and analysed using standard optical tools and the device uses much less energy than conventional modulators.

High performance and efficiency

The detector works at room temperature and Polzik says that it "promises performance comparable to the best cryogenically cooled electronics". "Moreover, the radio signals in our method are efficiently converted into optical signals, which can be transmitted via optical cables with much lower loss than electrical signals can be transmitted by metal wires," he says.

At the heart of the device is an antenna that is connected to a capacitor. One of the two capacitor plates is an extremely high-quality silicon-nitride membrane that is about 500 μm across and about 200 nm thick, after being coated with a reflective layer of aluminium.

When the capacitor encounters radio waves at its resonant frequency, the nanomembrane vibrates. "The radio waves detected by the antenna induce charge fluctuations in the capacitor," says Polzik. "By applying an external bias voltage to the capacitor, we can convert these fluctuations into mechanical vibrations of the membrane." A laser beam is bounced off the membrane, which produces an optical phase shift that can be measured using standard optical techniques. "We have thus converted a radio signal detected by the antenna into an optical signal," says Polzik.

Running hot

When traditional radio receivers pick up faint radio waves, heat-related noise can distort the signal. But when radio signals are converted into a resonant mechanical vibration, the random effect of heat becomes negligible. The reflected light picks out the radio wave with little of the noise that affects standard radio receivers.

The new device has a room-temperature sensitivity of 100 pV Hz–1/2 for radio waves at 1 MHz. The team expects that this could be improved by a factor of 20, which would put the receiver on a par with the best devices using cryogenics.

The next steps for the team are to use microfabrication techniques to further miniaturize the device so that it fits on a chip and to improve its sensitivity. "We also plan to extend the frequency range of the devices from a megahertz domain to the hundred megahertz to the gigahertz domain, which is most relevant for applications in communication and sensing," says Polzik.

A quantum internet

Potential applications of the detector include those that currently use cooled preamplifiers. These include high-resolution nuclear-magnetic-resonance systems and radio telescopes – both of which rely on liquid-helium-cooled detectors. Chip-sized devices could lead to smaller and more energy-efficient communication devices and navigation systems.

In the long term, the technology could make it possible to convert quantum states of microwave radiation into optical quantum states, claims Polzik. "Such a conversion will be an important step towards distributed quantum networks. It may help researchers to use optical photons – ideal carriers of quantum information – to connect distant superconducting qubits," he says.

Clear technological potential

Physicist Mika Sillanpää at Aalto University in Finland, who was not involved in the study, says that the research "has clear technological potential" to become reality in the future. "From the basic-research standpoint, the work creates a hybrid physical system, which has potential to function in the quantum-mechanical limit," he says.

Sillanpää adds that the technology could be used as a "router" or node to connect quantum computers. "At the moment this is mostly hype, but might become reality one day," he says.

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12 comments

Misleading title

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

Radio waves go in and light comes out, the headline is a perfect black box description of the device. I wrote the headline, by the way.

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

Radio waves go in and light comes out, the headline is a perfect black box description of the device. I wrote the headline, by the way.

No, because light is also going into the "black box". If you would measure the light power going in and the light power coming out, and compare both, you would see that even less light is coming out as going in (due to reflection loss). Therefore, in no way one can state that a radio wave is converted into light. The title is misleading!

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

Radio waves go in and light comes out, the headline is a perfect black box description of the device. I wrote the headline, by the way.

No, because light is also going into the "black box". If you would measure the light power going in and the light power coming out, and compare both, you would see that even less light is coming out as going in (due to reflection loss). Therefore, in no way one can state that a radio wave is converted into light. The title is misleading!

Yes, but what if the laser is part of the black box, as it would be in a practical device!

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

Radio waves go in and light comes out, the headline is a perfect black box description of the device. I wrote the headline, by the way.

No, because light is also going into the "black box". If you would measure the light power going in and the light power coming out, and compare both, you would see that even less light is coming out as going in (due to reflection loss). Therefore, in no way one can state that a radio wave is converted into light. The title is misleading!

Yes, but what if the laser is part of the black box, as it would be in a practical device!

The point is that the average power of the light is not influenced by the radio wave. There is always light coming out of the device, even if no radio wave is present. Only the light phase is modulated by a radio wave. I know that the title is misleading because it mislead me.

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

Radio waves go in and light comes out, the headline is a perfect black box description of the device. I wrote the headline, by the way.

No, because light is also going into the "black box". If you would measure the light power going in and the light power coming out, and compare both, you would see that even less light is coming out as going in (due to reflection loss). Therefore, in no way one can state that a radio wave is converted into light. The title is misleading!

Yes, but what if the laser is part of the black box, as it would be in a practical device!

I wouldn't say the title is misleading but perhaps a little awkward. Light is already coming in and is coupled to the radio waves. What the radio waves are converted into are modulations of the light, or more simply said into light signals.

Just detection and not conversion

From Nature's article:"The radio-frequency signals are detected as an optical phase shift with quantum-limited sensitivity". It is just the detecion of the radio signals via the phase shift of the reflected laser light from the vibrating membrane of the capacitor and not their conversion per se into light. The reference to the black box here is irrelavant.

According to the article the device converts radio signals to mechanical oscillations and not to light. That mechanical movement can modulate a light reflection is a rather mundane statement but used to great effect by the innovation described.

The Nature article has the correct title - so Katia is using journalistic freedom to describe something that is not, a trait of the boulevard press but not scientific reporting

Radio waves go in and light comes out, the headline is a perfect black box description of the device. I wrote the headline, by the way.

I was confused at first, when I read that radio waves were converted to light. That doesn't even make sense, radio waves are already light themselves.It should be 'Tiny membrane amplifies weak radio waves by laser modulation'.It's not 'converting' when the output energy is larger than the input energy, that's translating. Conversion would be if a membrane absorbed the radio wave energy and emitted a corresponding visible light photon, but this isn't the case here.

I was confused at first, when I read that radio waves were converted to light. That doesn't even make sense, radio waves are already light themselves.It should be 'Tiny membrane amplifies weak radio waves by laser modulation'.It's not 'converting' when the output energy is larger than the input energy, that's translating. Conversion would be if a membrane absorbed the radio wave energy and emitted a corresponding visible light photon, but this isn't the case here.

'Tiny membrane amplifies weak radio waves by laser modulation'. This suggests the device outputs a more powerful radio signal than it receives, which it doesn't!

The reason our headline emphasised "conversion" over "detection" is that the up-conversion of radio waves to light has technological relevance to radio astronomy and quantum computing. As Katia pointed out, there are other ways to detect weak radio signals.

With regards to absorption and emission: The membrane is an antenna and it does absorb the radio waves, that's what provides the energy for its oscillation.

As to whether the light is emitted from the membrane. Classical electrodynamics describes reflection as an absorption/emission process. So strictly speaking light with the characteristics of the radio wave is emitted from the membrane. However, I do take your point that the emission is very different from what occurs in other up-conversion schemes, where the higher frequency light is emitted by oscillating electrons that are excited by the lower-frequency light (in this case the radio wave).

CMB @ home

Luinguistic/political/journalistic freedoms aside, the novel part, the conversion of incident radio waves to mechanical vibration is an intriguing possibility from an engineering perspective - not so glitzy scientifically, but the commercial ultra low noise rf detection possiblities this approach opens up is astounding.

Not so sure about the contribution to quantum internet as claimed by the inventors, but ultra low noise, ultra low EM signal detection is currently very difficult due to the need for cryogenics, but this approach may make a swag of commercial ultra sensitive radio devices practical outside the lab.

Imagine, an ultra sensitive CMB detector device you can buy for $30? Put the telescope aside, fit the new device, along with with the optional ultra narrow field microwave lense to your equatorial mount and observe CMB on your TV.

In a practical, commercial device, I suspect the laser light phase shift detection will be the most complex aspect of it.